Battery-powered electric vehicle d.c. motor control

ABSTRACT

A battery powered electric vehicle d.c. motor control includes a first potentiometer actuable by an accelerator pedal and a second potentiometer actuable by a brake pedal. The motor current is controlled in accordance with the signals at the sliders of these potentiometers and contactors determine the connection mode of the motor under the control of a comparator which is sensitive to the signal at the slider of the first potentiometer only. This causes braking mode to be selected except when the accelerator pedal is depressed. A resistor is connected in series with the second potentiometer to ensure that a minimum braking demand signal is always present, but means is provided for cancelling this minimum braking demand signal in certain vehicle operating conditions and further means is connected to the comparator for overridingly selecting motoring mode in these conditions.

This invention relates to a battery-powered electric vehicle d.c. motor control.

It has already been proposed to determine whether the connection configuration of an electric vehicle d.c. motor is switched to provide motoring or braking in accordance with signals present on accelerator and braking demand potentiometers actuated by accelerator and brake pedals respectively. For example, in P.C.T. Application No. PCT/GB78/00046 there is disclosed a control circuit in which selection of the connection configuration is made by comparing the demand signals at the sliders of the two potentiometers.

The driving of a vehicle fitted with a circuit of this kind, however, does not correspond very closely to the driving of a petrol engined vehicle--in which substantial engine braking is obtained merely by releasing of the accelerator pedal, engine braking being independent of mechanical braking.

It is thus one object of the present invention to provide a battery-powered electric vehicle d.c. motor control in which braking can be initiated merely by releasing the accelerator pedal.

In accordance with the invention there is provided a battery-powered electric vehicle d.c. motor control comprising a first potentiometer actuable by an accelerator pedal, a second potentiometer actuable by a brake pedal, motor current control means connected to control the current flow in the motor in accordance with a demand signal determined by the signals at the sliders of said potentiometers, contactor means for varying the connection configuration of the motor to provide motoring or regenerative braking operation, contactor control means sensitive to the signal at the slider of the first potentiometer only so as to switch the contactors to provide braking mode except when the accelerator pedal is depressed out of its rest position, means for generating a minimum demand signal when both pedals are in their rest positions and means for overridingly causing the contactor control means to switch the contactors to motoring mode, and for cancelling said minimum demand signal in certain vehicle operating conditons.

Preferably, said means for generating a minimum demand signal comprises a resistor in series with the second potentiometer. The minimum demand signal can then be cancelled by means of a switch device connected across the series combination of the second potentiometer and said resistor.

An example of the present invention is illustrated in the accompanying drawings in which:

FIG. 1 is a block diagram identical to FIG. 1 of the drawings of the aforementioned application No. PCT/GB78/00046,

FIG. 2 is a circuit diagram showing a demand shaping circuit and a motor/brake comparator circuit used in FIG. 1,

FIG. 3 shows a modification of the part of the circuit shown in FIG. 7c of application No. PCT/GB78/00046 and,

FIG. 4 is a circuit diagram of an additional temperature sensitive circuit included in the system.

The block diagram of FIG. 1 is fully described in application No. PCT/GB78/00046 and will not be redescribed herein. In fact, although not shown in FIG. 1, two changes are made in the present case. Firstly, an added resistor 1001 is included in series with the brake potentiometer 18, and secondly, the motor brake comparator 19 does not have connections from the sliders of potentiometers 17 and 18 but has its inputs from the +ve end of potentiometer 17 and the slider of the same potentiometer.

These changes are shown in FIG. 2 which also show the demand speed shaping circuit 20 and the motor/brake comparator circuit 19.

The demand speed-shaping circuit includes a first input terminal C which receives its input from the collector of transistor P3 shown in FIG. 3 of application No. PCT/GB78/00046. This terminal is high whenever the main SCR of the chopper circuit is on. Terminal C is connected by two resistors R₁₀₀₂ and R₁₀₀₃ in series to the inverting input of an operational amplifier A₁₀₀₁, the non-inverting input of which is connected to the +8V positive supply rail by a resistor R₁₀₀₄. Connected between the inverting input of amplifier A₁₀₀₁ and its output terminal are a resistor R₁₀₀₅ and a capacitor C₁₀₀₁ in parallel.

The junction of the resistor R₁₀₀₂, R₁₀₀₃ is connected to the anode of a diode D₁₀₀₁, the cathode of which is connected to the collector of an npn transistor N₁₀₀₁ which has its emitter connected to the ground rail. A resistor R₁₀₀₆ connects the base of transistor N₁₀₀₁ to the ground rail and another resistor R₁₀₀₇ connects the base of that transistor to an input terminal E, which is connected to the forward/reverse selector switch (see FIG. 7a of application No. PCT/GB78/00046) so that terminal E is high whenever forward drive is selected.

The terminal C is also connected by a resistor R₁₀₀₈ to the inverting input of an operational amplifier A₁₀₀₂, the non-inverting input of which is connected by a resistor R₁₀₀₉ and a variable resistor R₁₁₀₀ in series to the +8V rail. The output of amplifier A₁₀₀₂ is connected by a resistor R₁₀₁₁ in parallel with a capacitor C₁₀₀₂ to the inverting input thereof and is also connected by a capacitor C₁₀₀₃ to the ground rail. Two resistors R₁₀₁₂ and R₁₀₁₃ in series are connected across the capacitor C₁₀₀₃ and a resistor R₁₀₁₄ connects the junction of these resistors to the base of a pnp transistor P₁₀₀₁. A diode D₁₀₀₂ has its cathode connected to output of amplifier A₁₀₀₁ and its anode connected to the base of transistor P₁₀₀₁.

Transistor P₁₀₀₁ has its base connected to the ground rail by a resistor R₁₀₁₅ and a variable resistor R₁₀₁₆ in series. The emitter of transistor P₁₀₀₁ is connected by a resistor R₁₀₁₇ to the +8v rail and its collector is connected by a resistor R₁₀₁₈ to the ground rail. An npn transistor N₁₀₀₂ has its base connected to the collector of the transistor P₁₀₀₁ and its emitter connected to the ground rail. The collector of the transistor N₁₀₀₂ is connected to the emitter of the transistor P₁₀₀₁. Transistors P₁₀₀₁ and N₁₀₀₂ operate as a very high input impedance compound emitter follower, with the emitter of transistor P₁₀₀₁ at a voltage one diode voltage drop above that at the base thereof. The accelerator pedal potentiometer 17 is connected across the collector emitter of the transistor N₁₀₀₂.

A further operational amplifier A₁₀₀₃ has its output terminal connected by a resistor R₁₀₂₀ to the base of the transistor P₁₀₀₁. This amplifier A₁₀₀₃ is connected as a voltage follower with its inverting input connected by a resistor R₁₀₂₁ to its output terminal and its non-inverting input connected by a resistor R₁₀₂₂ to the slider of a potentiometer R₁₀₂₃ which is connected at one end to the positive supply rail and its other end connected by a resistor R₁₀₂₄ to ground. Resistors R₁₀₂₂, R₁₀₂₃ and R₁₀₂₄ provide input bias current to amplifier A₁₀₀₃ to set its output at a predetermined value. The output of amplifier A₁₀₀₃ can be changed from this value in two ways viz by a signal from an operational amplifier A₁₀₀₄ (yet to be described in detail) the output of which is high when "motoring" mode is selected (as will be explained hereinafter) and by a signal from the field weakening amplifier (29 in FIG. 1). A capacitor C₁₀₀₄ connects to the output of amplifier to ground.

The brake pedal potentiometer 18 is connected at one end by the variable resistor R₁₀₀₁ to the anode of a diode D₁₀₀₃, the cathode of which is connected to ground, and at the other end by a resistor R₁₀₂₅ to the supply rail. The collector of an npn transistor N₁₀₀₃ is connected to said other end of the potentiometer 18 and its emitter is connected to ground. The base of transistor N₁₀₀₃ is connected to the collector of a pnp transistor P₁₀₀₂ and by a resistor R₁₀₂₆ to ground. The emitter of transistor P₁₀₀₂ is connected to the collector of transistor N₁₀₀₃. The base of transistor P₁₀₀₂ is connected by a variable resistor R₁₀₂₇ and a resistor R₁₀₂₈ in series to ground and by a pair of resistors R₁₀₂₉ and R₁₀₃₀ in series to the input terminal C. A capacitor C₁₀₀₅ connects the junction of the resistors R₁₀₂₉ and R₁₀₃₀ to ground. A further capacitor C₁₀₀₆ connects the base of the transistor P₁₀₀₂ to ground.

An operational amplifier A₁₀₀₅ has its inverting input connected by a resistor R₁₀₃₁ to the input terminal C. The non-inverting input of this amplifier A₁₀₀₅ is connected to the junction of two resistors R₁₀₃₂, R₁₀₃₃ which are connected in series between the supply rail and ground. A resistor R₁₀₃₄ and a capacitor C₁₀₀₇ in parallel connect the output terminal of amplifier A₁₀₀₅ to its inverting input terminal. A diode D₁₀₀₄ has its cathode connected to the output of amplifier A₁₀₀₅ and its anode connected to the base of the transistor P₁₀₀₂.

A resistor R₁₀₃₅ connects the output of the amplifier A₁₀₀₃ to the base of the transistor P₁₀₀₂.

The motor/brake comparator 19 of FIG. 1 is constituted by the amplifier A₁₀₀₄ already mentioned above. The inverting input of this amplifier A₁₀₀₄ is connected by a resistor R₁₀₃₆ to the anode of a diode D₁₀₀₅, the cathode of which is connected to collector of the transistor N₁₀₀₂. The anode of diode D₁₀₀₅ is also connected by a resistor R₁₀₃₇ to the supply rail. The non-inverting input of amplifier A₁₀₀₄ is connected by a resistor R₁₀₃₈ to the anode of a diode D₁₀₀₆, the cathode of which is connected to the slider of the accelerator pedal potentiometer 17. The anode of diode D₁₀₀₆ is also connected by a resistor R₁₀₃₉ to the supply rail. A feedback resistor R₁₀₄₀ is connected between the output terminal of amplifier A₁₀₀₄ and its non-inverting input and a capacitor C₁₀₀₈ is connected between the output terminal of amplifier A₁₀₀₄ and ground.

The resistors R₁₀₃₆ to R₁₀₃₉ are chosen so that the output of amplifier A₁₀₀₄ is high except when the slider of potentiometer 17 is in the bottom 5% of its travel, i.e. when the accelerator pedal is in its released position or only slightly depressed from that position.

For overriding the motor/brake comparator in certain conditions there is a pnp transistor P₁₀₀₃ which has its emitter connected to the supply rail and its collector connected by a resistor R₁₀₄₁ to the non-inverting input of amplifier A₁₀₀₄. When transistor P₁₀₀₃ is conductive the output of amplifier A₁₀₀₄ is high whatever the position of the accelerator pedal. The base of transistor P₁₀₀₃ is connected by a resistor R₁₀₄₂ to the supply rail and by a resistor R₁₀₄₃ to the collector of an npn transistor N₁₀₀₄. The emitter of the transistor N₁₀₀₄ is connected to ground and a resistor R₁₀₀₄ connects the base of this transistor to ground. The base of transistor N₁₀₀₄ is also connected by a resistor R₁₀₄₅ to the cathodes of three diodes D₁₀₀₇, D₁₀₀₈ and D₁₀₀₉, the anodes of which are connected to inputs from other parts of the control circuit. The cathodes of the diodes D₁₀₀₇, D₁₀₀₈ and D₁₀₀₉ are also connected by two resistors R₁₀₄₆ and R₁₀₄₇ in series to the ground rail and the junction of these resistors is connected to the base of an npn transistor N₁₀₀₅. A resistor R₁₀₄₈ connects the cathodes of diodes D₁₀₀₇, D₁₀₀₈ and D₁₀₀₉ to a terminal which is connected to a driver operable switch through which this terminal can be connected to the +8v supply rail, when it is required to inhibit regenerative braking. The emitter of transistor N₁₀₀₅ is grounded and its collector is connected to the collector of the transistor N₁₀₀₃. Whenever the signal at the anode of any one of the diodes D₁₀₀₇, D₁₀₀₈ or D₁₀₀₉ is high the transistors N₁₀₀₅, N₁₀₀₄ and P₁₀₀₃ turn on thereby shorting out the demand signal from the brake potentiometer 18 and forcing the output of amplifier A₁₀₀₄ high.

Turning now to FIG. 3 a bridge rectifier B₁₀₀₀ has its inputs connected to opposite ends of the motor armature winding. The negative output of the bridge rectifier is connected to one end of a transformer primary winding L₁₀₀₀. The positive output of the bridge rectifier is connected by a resistor R₁₀₅₀ to the cathode of a zener diode D₁₀₁₀, the anode of this zener diode being connected to the negative output of the bridge rectifier. A capacitor C₁₀₁₀ is connected across the zener diode D₁₀₁₀. A unijunction transistor U₁₀₀₀ has its B₁ terminal connected to the negative output of the bridge rectifiers B₁₀₀₀, its B₂ terminal connected by a resistor R₁₀₅₁ to the cathode of zener diode D₁₀₁₀ and its emitter terminal connected by a resistor R₁₀₅₂ to the cathode of zener diode D₁₀₁₀. A capacitor C₁₀₁₁ couples the emitter of u.j.t. U₁₀₀₀ to the other end of the winding L₁₀₀₀ and a diode D₁₀₁₁ is connected across the winding L₁₀₀₀ .

The secondary winding L₁₀₀₁ of the transformer is connected at one end to a +5v supply rail and at the other end to the cathode of a diode D₁₀₁₂ and the anode of a diode D₁₀₁₃, the cathode of which is connected to the +5v supply rail. A capacitor C₁₀₁₂ connects the anode of the diode D₁₀₁₂ to the +5v supply rail and becomes charged up whenever there is a voltage across the armature (generated by movement of the electric vehicle) sufficient to cause u.j.t. U₁₀₀₀ to oscillate.

A resistor R₁₀₅₃ connects the anode of the diode D₁₀₁₂ to the base of a pnp transistor P₁₀₀₄, the emitter of which is connected to the +5v supply rail. The base of the transistor P₁₀₀₄ is also connected to the junction of two resistors R₁₀₅₄ and R₁₀₅₅ connected in series between the output of an operational amplifier A₁₀₀₆ and the +5v supply rail. The amplifier A₁₀₀₆ has its non-inverting input connected to the junction of two resistors R₁₀₅₆ and R₁₀₅₇ connected in series between the +8v supply rail and ground and its inverting input connected to the junction of two further resistors R₁₀₅₈ and R₁₀₅₉ connected in series between ground and the cathode of a diode D₁₀₁₄, the anode of which is connected to the collector of the transistor P₁₀₀₄. A resistor R₁₀₆₀ is connected between the output of amplifier A₁₀₀₆ and its non-inverting amplifier so as to provide rapid switching thereof in the manner of a Schmitt trigger circuit. The output of amplifier A₁₀₀₆ is connected to the anode of the diode D₁₀₀₇ (FIG. 2).

The collector of the transistor P₁₀₀₄ is connected by a resistor R₁₀₆₁ to the base of an npn transistor N₁₀₀₆, which base is also connected by a capacitor C₁₀₁₃ to ground. The emitter of transistor N₁₀₀₆ is connected to ground and its collector is connected to a terminal n (which is the same as terminal n in FIG. 7c of application No. PCT/GB78/00046). The collector of transistor N₁₀₀₆ is also connected by a capacitor C₁₀₁₄ and a resistor R₁₀₆₂ in series to the base of a pnp transistor P₁₀₀₅, the emitter of which is connected to the +5v rail. The base of transistor P₁₀₀₅ is also connected to the junction of two resistors R₁₀₆₃ , R₁₀₆₄ connected in series across a capacitor C₁₀₁₅ connected at one side to the +5v rail and at the other side by a resistor R₁₀₆₅ and diode D₁₀₁₅ in series to a terminal p (which is the same as terminal p in FIG. 7c of application No. PCT/GB78/00046). The collector of transistor P₁₀₀₅ is connected by two resistors R₁₀₆₆ and R₁₀₆₇ in series to ground and the junction of these resistors is connected to the base of the transistor N₁₀₀₆. A diode D₁₀₁₆ and a resistor R₁₀₆₈ having no equivalent in FIG. 7c of application No. PCT/GB78/00046 connect the base of transistor N₁₀₀₆ to the collector of the transistor N₃₃ which is shown in FIG. 7b of application No. PCT/GB78/00046.

Turning finally to FIG. 4, the temperature sensitive circuit shown therein, includes two thermistors T₁₀₀₀, T₁₀₀₁ are arranged to be sensitive respectively to the temperatures of the motor itself and of a heat sink of the thyristor chopper circuit 10. Thermistor T₁₀₀₀ is connected at one side to ground and at the other side by two resistors R₁₀₇₀ and R₁₀₇₁ in series to the +8v supply rail. A pnp transistor P₁₀₀₆ has its base connected to the junction of these two resistors and its emitter connected to the +8v rail. The collector of the transistor P₁₀₀₆ is connected by two resistors R₁₀₇₂ and R₁₀₇₃ in series to ground. The junction of resistors R₁₀₇₂ and R₁₀₇₃ is connected to the anode of a diode D₁₀₂₀, the cathode of which is connected to said other side of the thermistor T₁₀₀₀. A capacitor C₁₀₂₀ is connected across the resistor R₁₀₇₃. The thermistor T₁₀₀₁ is connected in series with a resistor R₁₀₇₄ between the +8v rail and ground and a diode D₁₀₂₁ has its cathode connected to the junction of the resistor R₁₀₇₄ and the thermistor T₁₀₀₁, and its anode to the anode of diode D₁₀₂₀.

The junction of resistors R₁₀₇₂ and R₁₀₇₃ is connected by a resistor R₁₀₇₅ to the inverting input of an operational amplifier A₁₀₀₇ and by a resistor R₁₀₇₆ to the inverting input of an operational amplifier A₁₀₀₈. The non-inverting input of amplifier A₁₀₀₇ is connected to the +8v rail by the two resistors R₁₀₇₇ and R₁₀₇₈ is series selected so that the output of amplifier goes high if the temperature of either thermistor rises above 70° C. A positive feedback resistor R₁₀₇₈ is connected between the output and non-inverting input of amplifier A₁₀₀₇ and is of relatively high ohmic value so as to introduce a small amount of hysteresis into the switching action of the amplifier A₁₀₀₇ and to prevent the amplifier A₁₀₀₇ "dithering" between its two switch states when the temperature is at an almost steady value around 70° C.

The output of amplifier A₁₀₀₇ is connected by a resistor R₁₀₇₉ to the base of a npn transistor N₁₀₀₇. The emitter of this transistor N₁₀₀₇ is grounded and its collector is connected by two resistors R₁₀₈₀ and R₁₀₈₁ in series to a +12v rail. A pnp transistor P₁₀₀₇ has its emitter connected to the +12v rail its base connected to the junction of resistors R₁₀₈₀, R₁₀₈₁ and its collector connected via a warning lamp 1000 to ground. The lamp turns on whenever the output of amplifier A₁₀₀₇ goes high.

The amplifier A₁₀₀₈ has its non-inverting input connected by a pair of resistors R₁₀₈₂, R₁₀₈₃ in series to the +8v rail, and its output terminal connected by a resistor R₁₀₈₄ and a capacitor C₁₀₂₁ in parallel to its inverting input terminal. The output terminal of amplifier A₁₀₀₈ is connected by two resistors R₁₀₈₅ and R₁₀₈₆ in series to ground and also by two resistors R₁₀₈₇ and R₁₀₈₈ in series to ground. A npn transistor N₁₀₀₈ has its base connected to the junction of resistor R₁₀₈₅ and R₁₀₈₆ and its emitter connected by a resistor R₁₀₈₉ to ground. The collector of transistor R₁₀₀₈ is connected to the base of transistor P₁₀₀₁ (FIG. 2). Similarly an npn transistor N₁₀₀₉ has its base connected to the junction of resistors R₁₀₈₇ and R₁₀₈₈, its emitter connected by a resistor R₁₀₉₀ to ground and its collector connected to the base of transistor P₁₀₀₂ (FIG. 2).

The circuit described operates as follows: the d.c. signals on the upper ends of the accelerator and brake potentiometer 17 and 18 vary in accordance with the speed of the vehicle which is measured as a function of the mark-to-space ratio of the output of the Schmitt trigger circuit 14 (FIG. 1). In the case of the accelerator pedal potentiometer 17, the signal is boosted at low speed by the positive d.c. output of amplifier A₁₀₀₂ when the mark-to-space ratio is small, because of the relatively small positive average d.c. input to amplifier A₁₀₀₂. As the mark-to-space ratio increases to a point where the average d.c. input is larger than the bias input via the resistor R₁₁₀₀, the output of amplifier becomes increasingly negative so that the "extra" current provided by amplifier A₁₀₀₂ to the resistors R₁₀₁₆, R₁₀₁₅ which bias transistor P₁₀₀₁ is gradually reduced to zero. The signal then remains constant with increasing speed until the field weakening effect commences at which speed the signal at terminal a starts to fall and the output of amplifier A₁₀₀₃ falls linearly, thereby lowering the base voltage of transistor P₁₀₀₁ and consequently causing the signal at the upper end of potentiometer 17 to fall with increasing speed. The amplifier A₁₀₀₁ has an effect on the signal only when reverse drive is selected which causes the signal at terminal E to go low and turns off transistor N₁₀₀₁ which is normally on and holds the output of amplifier A₁₀₀₁ high, diode D₁₀₀₂ blocking this high output. When transistor N₁₀₀₁ turns off and the mark-to-space ratio of the signal at terminal C is high enough to cause the output of amplifier A₁₀₀₁ to start falling from its maximum value, diode D₁₀₀₂ pulls down the voltage on the base of transistor P₁₀₀₁ and reduces the maximum demand signal.

In the braking mode the signal at terminal C is inverted i.e. at low speeds the average voltage at terminal C is high. Thus the output of amplifier A₁₀₀₅ is low pulling the base of transistor P₁₀₀₂ low. As speed increases the voltage at terminal C decreases so that the output of amplifier A₁₀₀₅ increases until diode D₁₀₀₄ becomes reverse biased. The voltage at the base of transistor P₁₀₀₂ reaches a maximum value at this speed and then starts to fall until the voltage at terminal C is zero due to the falling voltage on capacitor C₁₀₀₅ (after which control of the motor is effected by field control) and as speed increases the output of amplifier A₁₀₀₃ starts to fall. In braking operation the output of amplifier A₁₀₀₄ is low (as will be explained) so that the "normal" maximum demand signal set by amplifier A₁₀₀₃ is lower than in motoring.

The state of amplifier A₁₀₀₄ is determined mainly by the position of the accelerator pedal. Thus when the pedal is in the first 5% of its travel braking is selected, whatever the position of the brake pedal. Depression of the accelerator pedal by more than 5% of its travel causes the output of amplifier A₁₀₀₄ to go high, so that motoring is selected. If, however, the signals at the anodes of any of the diodes D₁₀₀₇, D₁₀₀₈ or D₁₀₀₉ goes high, the transistors N₁₀₀₄ and P₁₀₀₃ turn on, thereby forcing the output of amplifier A₁₀₀₄ high whatever the position of the accelerator pedal. At the same time transistor N₁₀₀₅ turns on and shorts out the signal on the upper end of the brake pedal potentiometer 18 so that no demand signal is produced, as a result of the added resistor R₁₀₀₁, which sets the minimum brake current demand. This occurs when the amplifier A₁₀₀₆ (FIG. 3) output is high (indicating that the vehicle is stationary), during power up and when reverse has been selected.

Finally the circuit of FIG. 4 operates to illuminate the lamp 1000 if either the motor on the chopper circuit starts of become excessively hot. At a higher temperature both the maximum motoring demand and the maximum braking demand start to be reduced with increasing temperature until finally at a still higher temperature the maximum motor demand is reduced to zero. The maximum braking demand falls more slowly with rising temperature by appropriate choice of resistors R₁₀₉₀) so that when the output of amplifier A₁₀₀₈ is at its maximum value, the maximum brake demand is still not zero. Transistor P₁₀₀₆ is normally on at all temperatures, but if thermistor T₁₀₀₀ is open circuit transistor P₁₀₀₆ turns off so that the inverting input of amplifier A₁₀₀₈ goes low, so that its output is high, thereby setting the motoring and braking demands to zero or a low value. 

I claim:
 1. A battery-powered electric vehicle d.c. motor control comprising a first potentiometer actuable by an accelerator pedal, a second potentiometer actuable by a brake pedal, motor current control means connected to control the current flow in the motor in accordance with a demand signal determined by the signals at the sliders of said potentiometers, contactor means for varying the connection configuration of the motor to provide motoring mode operation or regenerative braking mode operation, contactor control means sensitive to the signal at the slider of the first potentiometer only so as to switch the contactors to provide braking mode operation except when the accelerator pedal is depressed out of its rest position, means for generating a non-zero minimum demand signal when both pedals are in their rest positions and means for overridingly causing the contactor control means to switch the contactors to said motoring mode operation, and for cancelling said minimum demand signal in certain vehicle operating conditions.
 2. A control as claimed in claim 1 in which said means for generating a minimum demand signal comprises a resistor in series with said second potentiometer.
 3. A control as claimed in claim 2 in which said means for cancelling said minimum demand signal comprises a switch device connected across the series combination of the second potentiometer and said resistor.
 4. A control as claimed in claim 3 in which said switch device is a semiconductor device which is rendered conductive when any said certain vehicle operating conditions occurs. 